1. Field of Invention
The present invention relates to a sheet brake for use in a printing press, and more specifically, to a sheet brake for a printing press for seizing a bottom portion of a printed sheet and guiding the sheet to a stack while stretching and decelerating the sheet.
2. Description of the Related Art
In a conventional printing presses, sheets are transported through a sheet-fed rotary printing press from a sheet feeder, through the printing cylinders, and, by way of a chain conveyor system to a sheet take-off device, which lays printed sheets off onto a stack. The printed sheets must be decelerated from a maximum speed to zero speed at a location of the sheet take-off device. The floating sheets must simultaneously be lowered onto the top of the stack of printed sheets.
There are problems not only in decelerating the sheets but also in that the sheets descend in a direction which is perpendicular to a major plane of each sheet and, as a result, the descending travel of the sheets is impeded by air resistance. This causes problems in laying off sheets at high speeds due to the oncoming sheet coming into contact with the rear edge of the sheet which has just been laid off.
Solutions to such problems in the form of blowers located above the stack have been used to force the laid-off sheet in a downward direction. The sheets, however, are powerfully affected by the streams of air and air resistance as described above. Precise sheet guidance is very difficult at high speeds, especially at the rear edge of the sheet. This is especially true when the rear corners of the sheet are forced upwardly by the streams of air coming from the cushion of air present below the sheet.
A sheet take-off apparatus for a rotary press is described in German OS 2 720 674. The sheet take-off apparatus has a sheet brake in the form of a box with several slots at the top on the printing press take-off device located near the rear edge of the stack of sheets. The box is adapted to blow air escaping through a first part of the box from slots arranged to direct air against the direction in which the sheets travel. Due to aerodynamic principles, lower-pressure currents are generated toward the box by other slots located at the top of the box. The resulting vacuum or suction allows a sheet which is secured by the conveyor gripper system and traveling past the top of the box to be captured via suction and held with a certain level of force. As the sheet travels on, the sheet skids over the box and, once released by the gripper system, is braked or decelerated by the force. The sheet can subsequently be guided at rest against stops in the vicinity of the take-off stack. For this purpose, the path traveled by the sheets or conveyor must slope down above the stack in the vicinity of the take-off device and in the direction of travel.
This is a major drawback because the above-described arrangement severely restricts the space available for the intervention of a possibly necessary sample-sheet extractor. To improve the downward motion of the taken-off sheet traveling from the conveyor to the top of the stack, the box or brake is provided with a stroke extending across the direction of travel. The sheet being laid off and secured by the gripper system is accordingly guided out of the direction of travel and drawn down to approximately the level of the top of the stack by aligning stops, where the sheet can be laid off.
This device has other drawbacks. Large and accordingly heavy sheets in particular cannot be braked or decelerated as desired, which is why the device has the extra, separately driven aligning stops at the rear edge of the stack to ensure that the sheets will directly encounter the stops at the forward edge of the sheet. Due to the blowing and suction action that occurs at the brake, the sheets must be held by the gripper until almost the last second to allow the sheets to be guided down accurately enough against the forward-edge stops. The downward-sloping sheet guidance is also a drawback. Otherwise, the above-described brake is also similar to what is called a sheet catcher. Sheet catchers are generally used to guide a sheet along its path and force the sheet into a prescribed direction, and the braking action is intended only to smooth the sheet along its path and not really intended to brake or decelerate the sheet, due to the priority of its guidance action.
Another sheet take-off device is described in DE-GM 80 03 052. It includes a sheet brake in the form of a combination suction cylinder and blowing-sucking device accommodated in a take-off device located upstream of the rear edge of a stack in the direction of travel and raised and lowered between a capture position and a lay-off position. Since the blowing-sucking device is rigidly fastened to the suction cylinder, the blowing-sucking device executes the rising and falling strokes together. The blowing-sucking device is intended to capture the sheets and feed them to the suction cylinder. This device has several is drawbacks related to how it handles the sheets being laid off. First, since the sheets are guided along the edges of the cylinder only at certain points, smooth and flat as possible contact cannot always be ensured in the case of large sheets. Again, the blowing-sucking device blows a current of air below the oncoming sheets, disturbing the sheet flow and preventing reliable capture of the sheets, especially large-format printed stock. The same approach also impedes flat take-off.
"Take-overs" are known as another means of facilitating the take-off of large sheets in particular. Take-overs accept the delivered sheets from the conveyor gripper system, decelerate the sheets, and guide the sheets against the forward-edge stops. The take-overs act only against certain points on the sheet, which can cause buckling and irregular braking. Furthermore, the sheets are exposed at high speeds to powerful deceleration within an unstable plane of orientation.